The invention will be described in terms of video signal processing though it is to be understood to be applicable to other signal processing environments wherein the signal to be processed contains repeating or redundant information.
Numerous video signal processing systems are designed to vary functionally in accordance with the signal-to-noise ratio (SNR) of the signal being processed. Examples of such systems are programmable bandwidth chrominance filters, horizontal peaking circuits and noise reducing recursive filters, to name a few. These systems typically have some parameter controlled by a signal corresponding to the noise level in the processed signal.
Designing a relatively accurate noise measuring apparatus to control such systems for use in consumer instruments such as a television receiver is a difficult undertaking. First and foremost it is not possible to discriminate between noise and actual signal in the average real time video signal. Second, since noise is random, the noise measurement should be the root-mean square of the noise. While algorithms are known for making root-mean-square noise measurements, the apparatus required to perform such algorithms are generally prohibitively complex or expensive for use in consumer products. Because of these difficulties, video system designers resort to estimating noise values.
The more common method of making noise estimates for video signals is to determine the average AC amplitude of portions of the video signal that do not contain video information such as the vertical blanking interval. The presumption is made that any AC variations in these portions of the signal arise from noise. The average amplitude values from respective measurements are integrated over time to produce more accurate results.
The noise measurements tend to be relatively static even if the integration time is short, e.g. several frame periods. Since noise often occurs in short bursts, which may encompass e.g. twenty five percent of the reproduced image, noise adaptive processing systems controlled by these static measurements cannot react to the noise bursts. In addition, the noise attendant portions of the video signal which do not contain video information may not be representative of the noise contained within the video information. This situation may occur where the source of the video signal is a storage medium which only stores active video and reconstructs the nonactive signal portions such as the vertical and horizontal blanking intervals.
Storey et al. in U.S. Pat. No. 4,249,210 disclose a method for measuring noise values from the active portions of video signals. The apparatus of the Storey et al. disclosure forms the differences of corresponding pixels from successive frames. If there is no interimage motion, the pixel differences contain only noise information. If there is interimage motion, the pixel differences contain both motion and noise information. In the Storey et al. system the presumption is made that the pixel differences having the smallest magnitude contain only noise information. Noise is discriminated from motion by selecting the smallest pixel difference values from each horizontal line of pixel differences and averaging these smallest pixel differences over a frame period. The averaged difference value is used as a noise estimate for the succeeding frame interval.
The Storey et al. noise measuring apparatus at least for stationary images will tend to produce a noise estimate value smaller than the actual average noise value. Secondly, while this system updates the noise estimate every frame period, it is not sufficiently dynamic to respond to a burst of noise within an image.
Ito et al. in the laid open UK Patent Application GB No. 2 102 651 A disclose a system wherein interimage pixel differences are squared and then averaged over an approximate frame period. Pixel differences with amplitudes greater than a predetermined threshold value are not included in the averages on the assumption that these differences include motion information. The accuracy of this noise measuring system depends on the selection of the predetermined threshold value. In actuality this value should be different for differing image contrast. The noise estimates provided by this system are not sufficiently dynamic to respond to short noise bursts.
It is an object of the present invention to determine relatively accurate noise estimates from active video signals representing a relatively small percentage of the reproduced image. It is a further object of the invention to provide noise estimates responsive to noise bursts, occurring in localized image portions.